Pre-implantation development in mammals involves the modulation of gene expression during cell differentiation. The sulfated glycoproteins of the murine zona pellucida constitute a gene family that is germ-line specific and active only during meiosis where its coordinate regulation occurs in the absence of cell division. During oogenesis, the zona pellucida appears as a discrete extra-cellular structure that surrounds the oocyte and contains three glycoproteins designated ZP-l, ZP-2 and ZP-3. We have developed and characterized a battery of rat monoclonal antibodies which are specific probes to the zona proteins. Five bind exclusively to Zp-2, three bind both to ZP-2 and ZP-3 and an additional two cell lilnes are specific for ZP-3. No mouse tissue other than the ovarian zonae pellucidae are immunoreactive with these antibodies and the zona proteins are antigenically distinct from other previously described extra-cellular matrix proteins. Although ZP-3 is thought to be the species-specific sperm receptor, monoclonal antibodies to either ZP-3 or to ZP-2 appear equally effective in inhibiting in vivo and in vitro fertilization. Parenterally administered antibody localizes to the zonae surrounding growing oocytes in the ovary. After ovulation, the bound antibody prevents the formation of the 2-cell embryo. This effect is reversible after 80 days and the resumption of fertility is associated with the disappearance of anti-zona antibody from the ovary. The presence of the antibodies does not perturb cleavage, the formation of the blastocyst or the hatching reaction. Using immunological probes we have recently screened a Lambda cDNA library made from polyA enriched RNA of murine ovaries. We have identified 51 positive clones and are in the process of characterizing them. These clones will be used in initial studies to determine the genomic organization of the zona genes and identify elements that may control their coordinate expression.